The present invention relates to a pattern formation method and apparatus for forming fine patterns used in the fabrication of semiconductor devices and, more particularly, to a method and apparatus for performing development and drying in forming such fine patterns by lithography.
Recently, with increasing scale of MOSLSIs, the chip sizes are increasing, and patterns in LSI fabrication are shrinking; nowadays, patterns having line widths of less than 100 nm are formed. This narrowing of lines results in the formation of patterns having high aspect ratios (height/width).
Also, the formation of fine patterns necessarily increases the aspect ratios of resist patterns as processing masks used in the etching process. These resist patterns can be formed by processing a resist film as an organic material by lithography. That is, when a resist film is exposed to light, the molecular weight or the molecular structure in the exposed region changes to produce a difference in solubility in a developer between this exposed region and the unexposed region. By using this difference patterns can be formed in the resist film by development. If this development continues, even the unexposed region starts dissolving in the developer and the patterns vanish. Therefore, rinsing is performed using a rinse solution to stop the development. Finally, the rinse solution is removed by drying to form resist patterns as processing masks in the resist film.
One major problem encountered when drying is performed in such fine pattern formation is the bending or distortion of patterns 1707 as shown in the sectional view of FIG. 17. That is, such fine resist patterns having a high aspect ratio are formed through rinsing and drying after development. High-aspect-ratio fine patterns are not restricted to resist patterns. For example, substrate patterns with a high aspect ratio are formed through cleaning, rinsing (washing), and drying after a substrate is etched using resist patterns as masks. During the drying after the rinsing process, the patterns 1707 bend toward each other. This phenomenon becomes conspicuous as the aspect ratio of the patterns 1707 increases. As shown in FIG. 18, this phenomenon is caused by a bending force (capillary force) 1810 exerted by a pressure difference between a rinse solution 1802 remaining between patterns 1801 and outside air 1803 when a resist or a substrate is dried. It is reported that this capillary force 1810 depends on the surface tension produced by the liquid/gas interface between the rinse solution 1802 and the patterns 1801 (reference: Applied Physics Letters, Volume 66, pp. 2655-2657, 1995). This capillary force not only bends resist patterns made from an organic material but also has power to distort even strong patterns made from, e.g., silicon, an inorganic material. This makes the aforesaid problem of the surface tension of rinse solution very important. This capillary force problem can be solved by processing using a rinse solution with small surface tension. For example, when water is used as a rinse solution, the surface tension of water is about 72 dyn/cm. However, the surface tension of methanol is about 23 dyn/cm. Therefore, the degree of pattern bending or collapse is suppressed more when water is replaced with ethanol and the ethanol is dried, than when water is directly dried. Furthermore, pattern bending is more effectively suppressed when the rinse solution is replaced with a perfluorocarbon solution and this perfluorocarbon solution is dried. However, as long as these liquids are used, pattern bending cannot be eliminated, although it can be reduced, because all of these liquids have surface tension to some extent.
To solve this problem of pattern bending, it is necessary to use a rinse solution with a zero surface tension or to first replace a rinse solution by a liquid having a zero surface tension and then dry this liquid. A supercritical fluid is an example of the liquid with a zero surface tension. This supercritical fluid is a gas at a temperature and a pressure exceeding the critical temperature and the critical pressure, respectively, and has solubility close to that of a liquid. However, the supercritical fluid has tension and viscosity close to those of a gas and hence can be said to be a liquid keeping a gaseous state. Since this supercritical fluid does not form any liquid/gas interface, the surface tension is zero. Accordingly, when drying is performed in this supercritical state, there is no surface tension, so no pattern bending takes place. Carbon dioxide is generally used as this supercritical fluid. Since carbon dioxide has low critical points (7.3 MPa, 31xc2x0 C.) and is chemically stable, it is already used as a critical fluid in biological sample observations.
Conventionally, supercritical drying using the supercritical state of carbon dioxide is done as follows. That is, liquefied carbon dioxide is previously introduced into a predetermined processing vessel to replace a rinse solution by repeatedly discharging the solution. After that, the processing vessel is heated to a temperature and a pressure higher than the critical points, changing the liquefied carbon dioxide in the vessel into supercritical carbon dioxide. Finally, while only this supercritical carbon dioxide adheres to fine patterns, the vessel is evacuated to vaporize the supercritical carbon dioxide and thereby dry the pattern.
Supercritical drying apparatuses marketed or manufactured so far to perform the supercritical drying as described above have the structures as shown in FIG. 19. In this supercritical drying apparatus, a carbon dioxide cylinder 1903 is. connected to a reaction chamber 1901 as a processing vessel for holding a substrate 1902 to be processed. A temperature controller 1910 controls the internal temperature of the reaction chamber 1901. In this supercritical drying apparatus, after supercritical carbon dioxide is supplied to replace a rinse solution, this supercritical carbon dioxide is exhausted at a given flow rate by a flow meter 1911. No pressure adjustment is performed during liquefaction and supercritical carbon dioxide processing. The pressure depends upon the amount of liquefied carbon dioxide. Therefore, the pressure after heating is increased to be much higher than the critical pressure by supplying liquefied carbon dioxide as much as possible. Additionally, to supply a sufficient amount of liquefied carbon dioxide, it is necessary to cool the reaction chamber 1901 to the extent that moisture aggregates.
Conventionally, this apparatus is used in resist pattern formation, particularly drying after rinsing, as follows. This drying method will be explained below. First, the substrate 1902 to be processed is rinsed and placed in the reaction chamber 1901. In this state, the rinse solution is still adhered on the substrate 1902. After that, a liquid of carbon dioxide is supplied from the cylinder 1903 into the reaction chamber 1901 heated to a predetermined temperature by the temperature controller 1910, thereby replacing the rinse solution. Next, the interior of the reaction chamber 1901 is set at a temperature and a pressure exceeding the critical points to convert the liquefied carbon dioxide in the reaction chamber 1901 into supercritical carbon dioxide. After that, this carbon dioxide as a supercritical fluid is exhausted from the reaction chamber 1901 to evacuate it, thereby vaporizing the supercritical carbon dioxide and drying resist patterns.
It is also possible to supply dry ice (solid carbon dioxide) into the reaction chamber without using a cylinder. In this method, supercritical carbon dioxide is generated in the reaction chamber by heating the dry ice in the reaction chamber.
Unfortunately, when these conventional supercritical drying apparatuses are used to dry after rinsing in resist pattern formation, resist patterns formed in a dried resist film swell and hence cannot be used as etching masks.
When drying is performed as above, in the reaction chamber the pressure of the supercritical carbon dioxide is about 10 MPa, and is sometimes about 12 MPa. If components other than the carbon dioxide exist around the resist film in the reaction chamber 1901 in this state, pattern swelling of the thin resist film occurs. More specifically, if moisture is condensed on the inner walls of the reaction chamber for forming supercritical carbon dioxide, the water thus formed is incorporated into pressurized supercritical carbon dioxide. When this high-pressure supercritical carbon dioxide diffuses into a thin resist film (organic material), the moisture incorporated into the supercritical carbon dioxide also diffuses into the thin resist film and is held inside it. Since this water contains carbon dioxide, this carbon dioxide dissolved in the water in the thin resist film vaporizes and abruptly increases the volume during drying, i.e., evacuation, thereby swelling the thin resist film.
The present inventors investigated the moisture in resist which causes resist pattern swelling and found that, as shown in FIG. 20, a resist contains a large amount of moisture in supercritical drying using the conventional supercritical drying apparatus. FIG. 20 is a graph showing the results (thermal desorption spectra) of analyses of gases of molecules (water molecules) having a mass number of 18 released from a thin resist film. A curve (a) in FIG. 20 indicates the result before supercritical drying, and a curve (b) indicates the result after supercritical drying using supercritical carbon dioxide at a pressure of 10 MPa. As is apparent from FIG. 20, the thin resist film contains a larger amount of water after supercritical drying than before that. That is, when supercritical drying is done by the conventional method, water is incorporated into a thin resist film.
As described above, when resist patterns are formed using a supercritical fluid by the conventional method, no fine patterns can be accurately formed owing to pattern swelling and the like.
It is, therefore, a principal object of the present invention to accurately form fine patterns by using a supercritical fluid without any pattern bending or pattern swelling.
To achieve the above object, according to one aspect of the present invention, a resist pattern layer having a predetermined pattern is formed from a resist film of an organic material formed on a substrate. A rinse process is performed by exposing the resist pattern layer to a rinse solution. Before the rinse solution sticking to the resist pattern layer dries out, the resist pattern layer is exposed to supercritical carbon dioxide having a pressure of 8.5 MPa or less. After that, the supercritical carbon dioxide is vaporized by lowering the pressure of the ambient of the substrate.
This arrangement suppresses the entrance of moisture into the resist pattern layer exposed to the supercritical carbon dioxide.
According to another aspect of the present invention, a pattern formation apparatus comprises a closable reaction chamber in which a substrate to be processed is placed, supply means for supplying supercritical carbon dioxide into the reaction chamber, pressure control means for controlling the internal pressure of the reaction chamber, and temperature control means for controlling the internal temperature of the reaction chamber.
With this arrangement, carbon dioxide already made supercritical is supplied into the reaction chamber.
According to still another aspect of the present invention, a resist pattern layer having a predetermined pattern is formed from a resist film of an organic material formed on a substrate. A rinse process is performed by exposing the resist pattern layer to a rinse solution. Before the rinse solution sticking to the resist pattern layer dries out, the resist pattern layer is exposed to a processing fluid not in gaseous state and having a predetermined density higher than in gaseous state or more. This processing fluid is a gas in steady state. Subsequently, the resist pattern layer is exposed to a supercritical fluid. After that, the supercritical fluid is vaporized by lowering the pressure of the ambient of the substrate.
With this arrangement, the rinse solution is replaced by the processing fluid and removed from the resist pattern layer. Also, the processing fluid is replaced by the supercritical fluid and removed from the resist pattern layer.
According to still another aspect of the present invention, a pattern formation apparatus comprises a closable reaction chamber in which a substrate to be processed is placed, first supply means for supplying, into the reaction chamber, a processing fluid not in gaseous state and having a predetermined density higher than in gaseous state or more, second supply means for supplying a supercritical fluid into the reaction chamber, pressure control means for controlling the internal pressure of the reaction chamber, and temperature control means for controlling the internal temperature of the reaction chamber, wherein the processing fluid is a gas in steady state.
With this arrangement, the processing fluid not in gaseous state and having a density higher than in gaseous state and the supercritical fluid are not generated in but supplied into the reaction chamber.
According to still another aspect of the present invention, a resist film of an organic material formed on a substrate is exposed to light. A solvent having developing properties is added to a processing fluid not in gaseous state and having a density higher than in gaseous state, a density at which the solvent uniformly mixes or more is set, and development is performed by exposing the exposed resist film to the processing fluid, thereby forming a resist pattern layer having a predetermined pattern on the substrate. This processing fluid is a gas in steady state. The resist pattern layer is exposed to a supercritical fluid having a pressure equal to or less than the pressure of the processing fluid. After that, the supercritical fluid is vaporized by lowering the pressure of the ambient of the substrate.
With this arrangement, after development is performed by the solvent contained in the processing fluid, this processing fluid is replaced by the supercritical fluid and removed from the resist pattern layer. In this way, development is stopped.
According to still another aspect of the present invention, a resist film of an organic material formed on a substrate is exposed to light. The exposed resist film is developed by exposing it to a polar processing fluid not in gaseous state and having a density higher than in gaseous state, thereby forming a resist pattern layer having a predetermined pattern. This processing fluid is a gas in steady state. The resist pattern is then exposed to a supercritical fluid. After that, the supercritical fluid is vaporized by lowering the pressure of the ambient of the substrate.
With this arrangement, after development is performed using the polar processing fluid, this processing fluid is replaced by the supercritical fluid and removed from the resist pattern layer. In this manner, development is stopped.